Superconductive materials are those where the electric resistance falls to zero (10.sup.-20 Ohm/cm) below a critical temperature, its value depending on the material. The superconductivity is defined only within a critical surface (only figure), its axes being the temperature, the electrical current and the magnetic field. Thus, for a given working temperature there is a defined curve of critical current in function of the magnetic field generated and/or applied to the superconductor.
The main practical superconductor materials are NbTi and Nb.sub.3 SN. Their working temperature is only 4.2K, which is the boiling temperature of liquid helium. This is the main limitation to large scale application of superconductivity. Superconductors are being used almost exclusively for winding of magnets. Manufactured from wires (NbTi and Nb.sub.3 Sn) or tapes (Nb.sub.3 Sn) with high critical current densities (3500 A/mm.sup.2 at 5 Tesla for NbTi), they allow the winding of compact magnets for the production of high fields (up to 18 Tesla) in large volumes.
The most popular superconductor magnets are the ones used for the formation of medical images by nuclear magnetic resonance (MRI) and for materials analysis by the same principle (NMR), the magnets for ore separation and research magnets for high fields. Among the latter are the magnets used in large particle accelerators (SSC, HERA, KEK, etc.).
The superconductor magnets are included in the conceptual project of large machines, among them nuclear fusion reactors, energy generators for magneto-hydro-dynamics (MHD) and magnets for energy storage in space stations. Of course, some difficulties in the feasibility of these large machines transcend superconductivity.
The oxide superconductors of high critical temperature were discovered in 1986. These are intermetallic compounds involving copper oxide and rare earths, with perovskite (mica) crystal structure. Their critical temperatures vary from 30K to room temperature and their critical fields are above 60 Tesla. Therefore these materials are considered very promising and may replace with advantages the Nb.sub.3 Sn and NbTi in the manufacture of magnets and find other applications not feasible with liquid helium, such as transmission of electricity, for example. These materials are not yet on the market as wires, cables, films, tapes or sheets.